1. Field of the Invention
The embodiments herein generally relate to wireless communication, and particularly relates to a method of optimizing Closed Subscriber Group (CSG) selection in wireless communication.
2. Description of the Related Art
A Closed Subscriber Group identifies subscribers of an operator who are permitted to access one or more cells of the PLMN but which have restricted access. In automatic mode of CSG selection, the access stratum utilizes the White List in a UE (combination of Operator CSG list and Allowed CSG list) to re-select/handover to a CSG cell. In Manual mode, the available CSGs are scanned and displayed to user based on configuration by HPLMN on the UICC. Upon manually selecting a CSG cell, the UE behavior is differentiated based on whether the selected CSG belongs to the registered PLMN or not. If the PLMN of the selected CSG is different than the currently registered PLMN, then the MS performs the following:
MS saves the “current PLMN selection mode” and RPLMN into a non-volatile memory
MS moves to a MANUAL PLMN selection mode.
Upon successful registration on the registered CSG cell, RPLMN is not updated with that of the selected PLMN.
If the registration fails or upon loss of coverage on the user selected CSG, MS should change the PLMN selection mode to one saved prior to MANUAL CSG selection and attempt recovery with the RPLMN.
The following case scenarios describes the ambiguities in the current specification
Case 1: Not storing the selected CSG as RPLMN upon successful manual CSG selection as specified in section 5.5.4 of 22.220, can lead to UE context mismatch between the MS and MME or SGSN during recovery of normal service using the stored RPLMN as shown in FIG. 1. Specification clearly mentions that the current PLMN shall not be stored as RPLMN so the EF's_EF_EPSLOCI, EF_PSLOCI, EF_LOCI shall not be updated on to the UICC by the MS. However, there is no mention about other values which are updated on UICC like EF_EPSNSC, EF_Keys, EF_KeysPS etc. When UE attempts recovery due to registration failure or MS is no longer in the coverage of the CSG, the MS shall use wrong parameters with the target MME/SGSN during registration e.g. GUTI, Old Lai, Last registered TAI etc, also will not match at the target MME since network would have deleted the related registration info during successful registration of the CSG.
Case 2: The current specification doesn't consider subsequent manual user re-selection of CSG cells. It is specified that a MS should change the PLMN selection mode to MANUAL if the selected CSG ID does not belong to the current RPLMN. Also the RPLMN should not be updated during a successful manual CSG selection. During a subsequent MANUAL CSG selection, the following points such as what value the MS should consider as RPLMN for the PLMN comparison, the actual RPLMN or the currently selected PLMN of the CSG are ambiguous. In case the selected CSG ID is different from both RPLMN and the currently selected CSG cell, saving of the current PLMN selection mode as per clause i) of section 4.4.3.1.3.3 of 23.122, which is MANUAL due to the previous manual CSG selection, will overwrite the previously saved PLMN selection mode during the first manual CSG Selection as shown in FIG. 2. This may cause UE to go in to an unintended PLMN selection mode while attempting recovery as per clause iv) of 4.4.3.1.3.3 of 23.122.
Case 3: The current specification doesn't consider a UE undergoing a power cycle when camped on a manually selected CSG cell as shown in FIG. 3. If the PLMN selection mode is changed to MANUAL from AUTOMATIC due to manual CSG selection as per clause II), 4.4.3.1.3.3 of 23.122 and UE is powered off/restarted in this state, MS shall try for normal service in MANUAL PLMN selection mode with the Registered PLMN. In case the MS has deleted the RPLMN or has no valid RPLMN, MS may stay in limited service mode until user manually selects a PLMN or changes the mode to AUTOMATIC.
Case 4: Assuming there are multiple white listed CSG cells in the vicinity of the MS and user selects the less suitable CSG cell manually, the current method adopted by the specification requires a manual CSG selection every time the MS is restarted even though the MS stays in the same CSG area. Upon restart, MS shall directly camp on to the more suitable CSG cell automatically thus requiring the user to reselect the required CSG cell manually as shown in FIG. 4. This kind of scenario is easily possible in a region with overlapping CSG cells where the MS is subscribed to both CSGIDs.
In view of the foregoing, it is realized that the current method for Manual CSG selection in a PLMN different than the current registered PLMN is ambiguous, which can cause the MS to go into an undesired PLMN state. Further the existing method is not optimal causing extra NAS signaling on the UE and the network during power on. Hence there is a need for a method and system for addressing the existing loop holes and to optimize the manual CSG selection.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent upon a reading of the specification and a study of the drawings.